Refractory for production of aluminum by electrolysis of aluminum chloride

ABSTRACT

SELECTIVE INTERFACIAL BOUNDING OF MOLTEN METAL CHLORIDE ELECTROLYTE BY NITRIDE-BASED REFRACTORY MATERIAL IN CELLS FOR PRODUCTION OF ALUMINUM AND CHLORINE BY ELECTROLYSIS OF ALUMINUM CHLORIDE.

S, 39?@ 5 C, JACOBS REFRACJORY FOR PRODUCTION OF ALUMINUM BY ELECTROLYSIS OF ALUMINUM CHLORIDE Filed Sept. 9, 1971 3,785,941 REFRACTORY FOR PRODUCTION OF ALUMINUM BY ELECTROLYSIS F ALUMINUM CHLORIDE Stanley C. Jacobs, Lower Burrell, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa. Filed Sept. 9, 1971, Ser. No. 178,895 Int. Cl. CZZd 3/02, 3/12 U.S. Cl. 204--67 21 Claims ABSTRACT OF THE DISCLOSURE Selective interfacial bounding of molten metal chloride electrolyte by nitride-based refractory material in cells for production of aluminum and chlorine by electrolysis of aluminum chloride.

This invention relates to the production of aluminum by electrolysis of aluminum chloride and, more particularly, to the avoidance of bath contamination by the selective interfacial bounding of molten metal chloride electrolyte by nitride-based refractory material in an electrolytic cell for production of aluminum from aluminum chloride.

While the production of aluminum by electrolysis of aluminum chloride has been a long-desired and theoretically feasible objective of the art, the economic attainment thereof has never become an economic reality. Among the many reasons therefor are numerous unsolved problems occasioned, for example, by the highly corrosive nature of the alkali or alkaline earth metal chloride electrolyte components. Moreover, the vapors or gases emanating from the electrolysis, as well as the complex salts or eutectics of the bath components and the products of electrolysis, all of which will be herein broadly encompassed by the term electrolyte, are of corrosive character and apparently compound the problem. Among such problems are the short life of cell components and the detrimental contamination of the bath through reaction thereof with the confining environmental elements in the electrolytic cells. In more particularity, the electrolyte tends to penetrate and react with conventional refractory materials heretofore known for use in electrolytic reduction cells. Many refractories comparatively insensitive tofluoride electrolytes such as used in electrolysis of alumina are highly sensitive to chloride-based electrolytes and to the chlorine formed in electrolysis of aluminum chloride. One of the consequences of such reactive deterioration of the refractories by the electrolyte in electrolysis of aluminum chloride is the formation of a sludge within the cell. The presence of such sludge not only progressively decreases the efficiency of operation of cells used for electrolytic production of aluminum from aluminum chloride, but its rapid formation also requires shut-down at frequent intervals for cleaning of the cells and removal of sludge, with consequent debilitation of economic production operations. By way of example, refractories conventionally employed in Hall-type reduction cells, such as silica and silica-based refractories, alumina or alumina-based refractories and even nitride-bonded silicon carbide, as opposed to the nitride-based refractories employed according to the present invention, have proved to have only limited resistance to attack by the electrolyte. In fact, such prior art refractories pose additional problems because of their partial solubility, particularly in that the oxygen Values therein operate to consume carbon from the anode, forming carbon dioxide and carbon monoxide, which additionally detrimentally affect the efficient production of aluminum from aluminum chloride, and, in the case of a silicabased refractory, silicon from the refractory contaminates the aluminum being produced, and such use of prior art United States Patent O "ice 3,785,941 Patented Jan. 15, 1974 refractories tends to promote corrosion of the refractory by such aluminum. p

In accordance with the principles of this invention, a substantial number of the above-mentioned problems attendant electrolysis of aluminum chloride may be materially reduced, if not largely obviated, by selectively interfacially bounding the electrolyte with nitride-based refractory material in contact therewith. By nitride-based refractory material I mean a refractory material having a nitride as its base, either alone, or associated with, for example, as a mixture or in compound or combined form, an oxide of silicon, boron or aluminum, and wherein the nitrogen concentration of the nitride is between about 25% and about 60% by weight of the nitride. Preferred base nitrides are those of silicon, boron and aluminum. However, other nitrides such as those of titanium, chromium, hafnium, gallium, zirconium and the like are also useful according to the invention. Presently preferred nitride-based materials usefully employable in accordance with the principles of this invention include silicon oxynitride, silicon nitridebonded fused silica, silicon nitride, aluminum nitride and boron nitride. Other materials which may be employed include, for example, nitride-based materials wherein the metal ion is aluminum, boron or silicon. Silicon oxynitride which is available commercially, for example, as Norton Silicon Oxynitride LON-4072, has the general formula Si2ON2 but is sometimes considered as a combination of silicon nitride and silica, specifically, Si3N4 and SiO2. Likewise silicon nitride-bonded fused silica is commercally available under the Carborundum trademark of Refrax- FS.

The above named refractory materials employable in the practice of this invention may be preformed by molding or the like or may be formed in place as a cell liner by slip casting, hot pressing or the like, or by positioning t0- gether in block form after being cut from larger blocks thereof. When used as a cell liner, such refractories may be used as a continuous liner, as separate blocks adjacent one another, as separate blocks joined together by a suitable corrosion-resistant adhesive, or as layers of blocks. Such materials are also useful according to the invention for cell structures other than linings, such as for spacers which support the individual electrode elements in bipolar electrode assembly within a reduction cell, as well as for sarnpling tubes, or to line openings in the cell. In general, such refractories are useful for most, if not all, of the non-conducting means that interfacially bound the electrolyte within the cell.

DESCRIPTION OF THE DRAWING For a better understanding of the invention, reference will now be made to the appended drawing, which forms a part hereof and is schematically illustrative of a presently preferred embodiment of an electrolytic cell incorporating the principles of this invention.

Aluminum chloride is introduced through an opening 10, either continuously or at intervals, into a molten bath of metal halide 16 confined within an electrolytic cell 12, bounded by `a perimetric metal shell 1'4. The metal halide bath .16 desirably constitutes -molten alkali metal chlorides, as for example, a molten salt mixture of sodium chloride, lithium chloride and aluminum chloride, the sodium and lithium chloride forming most of the electrolyte. According to the principles of this invention, the interior of the cell is lined with refractory material 18 of the character specified above, and such refractory material is also used as one or more spacers or the like 20 separating one or more bipolar electrodes 22, for example, positioned intermediate an anode 24 and a cathode Z6, both of which may be of graphite, and for those other non-electrically conducting parts of the cell which are disposed in interfacial bounding contact with the electrolyte as defined hereinabove. When current is passed from an external source (not shown) via an electrically conductive lead 27 t0 the cathode 26 and thence through the bath to the anode 24, the aluminum chloride content of the bath is converted to metallic aluminum Z8, which accumulates at the bottom of the cell, and chlorine gas 30, which moves from the lower or positively charged surfaces of the bipolar electrode elements such as 22 toward anode 24 and the top of the cell, and which, in its elemental form, will contact at least some of the refractory surfaces within the cell. The chlorine gas is readily removed from the cell through an appropriate opening 32, and aluminum may be tapped through a suitable orice (not shown) or siphoned out of the cell.

DESCRIPTION OF THE PREFERRED EMBODIMENT By way of illustrative example, aluminum has been produced in a multicompartment cell employing silicon oxynitride as a refractory lining and as spacers between bipolar electrodes therein, which cell was operated substantially continuous-ly at about 700 C. The molten halide bath consisted essentially of approximately equal parts by weight of sodium chloride and lithium chloride and included about 6% by Weight of aluminum chloride dissolved therein. Aluminum chloride was fed into the bath substantially continuously to replace the aluminum chloride electrolyzed therein. The cell was operated continuously for 120 days at about 2.7 volts per compartment with no ascertainable contamination of the bath or decrease in eciency of operation. Substantially the same procedure was used using silicon nitride-bonded fused silica as the refractory material in place of the silicon oxynitride with equivalently beneficial results.

While this invention has been described in terms of preferred embodiments, the claims appended hereto are intended to encompass all embodiments which fall within the spirit of the invention.

Having thus described my invention and certain embodiments thereof, I claim:

1. A cell for the production of aluminum by the electrolysis of aluminum chloride, said cell having an electrolytic chamber for holding a bath of molten metal chloride-based electrolyte having dissolved therein aluminum chloride for electrolytic conversion to aluminum in said chamber, the sa-id chamber having a non-conducting interfacial bounding for said bath, for vapors or gases emanated from said bath, or both, formed of refractory material consisting essentially of a nitride, alone or associated with as a mixture or compound or in combined form an oxide of silicon, boron or aluminum.

2. The combination of claim 1 wherein the nitrogen content of the refractory material amounts to from about 25% to about 60% by weight of the nitride.

3. The combination of claim 1 wherein the refractory material consists essentially of a nitride of silicon, boron or aluminum or a nitride of silicon, boron or aluminum in association with an oxide of silicon, boron or aluminum.

I4. The combination of claim v1 wherein the refractory material is selected from the group consisting of silicon oxynitride, silicon nitride-bonded fused silica, silicon nitride, aluminum nitride and boron nitride.

'5. The combination of claim 1 wherein the refractory material consists essentially of silicon oxynitride.

6. The combination of claim 1 wherein the refractory material consists essentially of silicon nitride-bonded fused silica.

7. The combination of claim 1 wherein the refractory material consists essentially of silicon nitride.

S. The combination of claim 1 wherein the refractory material consists essentially of aluminum nitride.

9. The combination of claim 1 wherein the refractory material consists essentially of boron nitride.

10. The combination of claim 1 wherein the non-cohducting interfacial bounding comprises an innermost cell liner.

11. The combination of claim 1 wherein the non-conducing interfacial bounding comprises spacers between electrode elements.

12. The combination of claim 1 wherein the non-conducting interfacial bounding lcomprises an innermost cell liner and a portion in juxtaposition with at least one bipolar electrode positioned between an anode and a cathode in the cell.

13. In the production of aluminum by electrolysis of aluminum chloride dissolved in a molten metal chloridebased electrolyte, the step of providing non-conducting interfacial bounding for said aluminum chloride-containing metal chloride-based electrolyte, for vapors emanated from said electrolyte, or both, by refractory material consisting essentially of a nitride, alone or associated with as a mixture or in compound or in combined form an oxide of silicon, boron or aluminum.

14. The improvement of claim 13 wherein the nitrogen content of the refractory material amounts to from about 25% to about 60% by weight of the nitride.

15. The improvement of claim 13 wherein the refractory material consists essentially of a nitride of silicon, boron or aluminum or a nitride of silicon, boron or aluminum in association with an oxide of silicon, boron or aluminum.

16. The improvement of claim 13 wherein the refractory material consists essentially of a refractory material selected from the group consisting of silicon oxynitride, silicon nitride-bonded fused silica, silicon nitride, aluminum nitride and boron nitride.

17 The improvement of claim 13 wherein the refractory material consists essentially of silicon oxynitride.

`18. The improvement of claim 13 wherein the refractory material consists essentially of silicon nitride bonded fused silica.

19. The improvement of claim 13 wherein the refractory material consists essentially of silicon nitride.

20. The improvement of claim 13 wherein the refractory material consists essentially of aluminum nitride.

2/1. The improvement of claim 13 wherein the refractory material consists essentially of boron nitride.

References Cited UNITED STATES PATENTS 3,554,893 l/l97l De Varda 204-244 3,616,438 10/ 1971 Foley et al. 204-245 2,919,234 12/1959 Slatin ..1.. 204--67 3,029,194 4/1962 De Varda 204--244 X 2,915,442 12/ 1959 Lewis t 204-243 R 3,428,545 2/ 1969 Johnson 204-243 R FOREIGN PATENTS 542,886 1/ 1942 Great Britain 204--243 R JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner U.S. C1. XJR. 

